1. Field of the Invention
The present invention generally relates to a channel selecting circuit in a television receiver set and, more particularly, to a method for the adjustment of the frequency of a television channel in a television receiver tuning apparatus utilizing a phase locked loop (PLL) circuit.
2. Description of the Prior Art
The prior art pertinent to the present invention wil be discussed with the aid of FIG. 2 of the accompanying drawings which illustrates, in a block circuit diagram, a tuner employing a frequency synthesizer system utilizing a PLL circuit. In FIGS. 2, reference numeral 1 represents an antenna connected with a mixer 3 through an RF amplifier 2. The mixer 3 is in turn connected through an IF amplifier 4 with a video signal tuning circuit 5 (hereinafter referred to as an AFT circuit) comprised of a video signal detector and an automatic fine tuning (AFT) circuit 5, which AFT circuit 5 is in turn connected through a signal processing circuit 6 with a cathode ray tube 7 or any other suitable display unit. Reference numeral 8 represents a local oscillator, reference numeral 9 represents a programmable frequency divider, reference numeral 10 represents a phase comparator, reference numeral 11 represents a low pass filter, reference numeral 12 represents a microcomputer, reference numeral 13 represents a phase locked loop (PLL) circuit, reference numeral 14 represents a sync signal detecting circuit, reference numeral 15 represents an analog-to-digital (A/D) converter, reference numeral 16 represents a keyboard input circuit, an reference numeral 17 represents a keyboard having tens keys. In FIG. 3 reference numeral 18 represents channel up-shift and down-shift keys, reference numeral 19 represents a mode selector switch assembly including, among other switches, an automatic fine tuning (AFT) switch 19a, and reference numeral 20 represents fine frequency adjusting keys.
The system is so designed as to operate in the following manner. The keyboard input circuit 16 is comprised of, as best shown in FIG. 3, a keyboard matrix circuit connected with input and output ports of the microcomputer 12.
The operation which takes place when the AFT switch 19a is closed, that is, the automatic tuning operation, will now be described with reference to FIG. 2 and also to a flowchart shown in FIG. 4.
Assuming that the AFT switch 19a is closed and a certain channel is subsequently selected by the manipulation of one of the tens keys 17 or one of the channel up-shift and down-shift keys 18, the microcomputer 12 reconstructs the channel number allocated to the channel selected and calculates a data N (division ratio) for the programmable frequency divider 9 which corresponds to the frequency allocated to each channel, the microcomputer 12 subsequently setting the programmable divider 9 to the data N. The phase locked loop circuit 13 comprised of the programmable divider 9, the phase comparator 10, the low pass filter, and the local oscillator 8 is so locked as to cause the local oscillator 8 to oscillate at a frequency equal to a predetermined frequency set for the selected channel. On the other hand, a television signal received at the antenna 1 and amplified by the RF amplifier 2 is mixed at the mixer 3 with the output signal from the local oscillator 8, and then the IF amplifier 4 amplifies the mixed signal which is in turn applied to the AFT circuit 5 at which the frequency of the received signal applied from the IF amplifier 4 is discriminated. After the discrimination of the frequency of the received signal, the AFT circuit 5 outputs an AFT voltage varying so as to depict a curve generally similar to the shape of a figure "S" (i.e., the voltage abruptly increasing in the vicinity of a certain frequency and then decreasing), the detected signal being, after having been processed in the signal processing circuit 6, applied to the cathode ray tube 7 for the reproduction of a color television picture.
On the other hand, the AFT voltage outputted from the AFT circuit 5 is also supplied to the microcomputer 12 through the A/D converter 15 and is then compared with a predetermined voltage to determine if the AFT voltage is within a predetermined range. If the AFT voltage so compared is within the predetermined range, the automatic tuning operation is completed.
However, if the AFT voltage is not within the predetermined range, a decision is then made to determine whether the AFT voltage is higher than the predetermined range or whether the AFT voltage is lower than the predetermined range. In the event that the AFT voltage is higher than the predetermined range, the preset data N of the programmable divider 9 is incremented by .DELTA.N so that the oscillating frequency of the local oscillator 8 can be increased by .DELTA.f. On the other hand, in the event that the AFT voltage is lower than the predetermined range, the preset value N of the programmable frequency divider 9 is decremented by .DELTA.N so that the oscillating frequency of the ocal oscillator 8 can be decreased by .DELTA.f. This program flow is repeated until the AFT voltage falls within the predetermined range, thereby completing the automatic tuning operation wherefore a television receiver set receives the transmitted television signal at a frequency appropriate for the AFT voltage to fall within the predetermined range.
It is, however, to be noted that during this automatic tuning operation the fine frequency adjusting keys 20 do not work.
The manual tuning operation which is carried out during the opening of the AFT switch 19a will now be described with reference to FIG. 2 and also to a flowchart shown in FIG. 5.
Until the programmable divider 9 is set to the data N of the selected channel, the program flow proceeds in a manner similar to the automatic tuning operation shown in and described with reference to FIG. 4, even during the manual tuning operation, and, accordingly, the television receiver set is placed ready to receive the television signal at a frequency equal to a predetermined frequency oscillated by the local oscillator 8. However, when any one of the +F or -F fine tuning keys 20 is closed, a decision is then made to determine which one of the tuning keys 20 has been closed. If the fine tuning key 20 is the +F tuning key, the data N is incremented by .DELTA.N, but if it is the -F tuning key, the data N is decremented by .DELTA.F. A viewer of the television receiver set depresses either the +F tuning key or the -F tuning key through a number of times required to permit the television receiver to be in condition ready to receive the desired channel, thereby completing the manual tuning operation.
The prior art apparatus so constructed as hereinabove discussed is such that, when one of the automatic tuning mode and the manual tuning mode is selected, the microcomputer executes one of the program flows shown respectively in FIGS. 4 and 5, which is appropriate to the selected mode. Accordingly, where a fine frequency adjustment is desired to be effected even after a particular channel has been properly selected under the automatic tuning mode, depending on the source of the signal desired to be received and/or the intensity of the electric field of the signal of the selected channel, the automatic tuning mode is required to be switched over to the manual tuning mode in which the frequency being received can be manually adjusted to the frequency allocated to the selected channel in readiness for the manual tuning operation to be carried out by manipulating one of the fine tuning keys 20 to place the television receiver set in condition to receive the proper frequency. Thus, with the apparatus according to the prior art, the fine frequency adjustment requires relatively complicated and time-consuming procedures.